1. Field of the Invention
The present invention relates to a method for forming a magnetic pattern in a magnetic recording medium used for a magnetic recording device, method for producing a magnetic recording medium, a magnetic recording medium and a magnetic recording device.
2. Discussion of Background
A magnetic recording device represented by a magnetic disk device (a hard disk drive) has widely been used as an external memory device for an information processing device such as a computer, and has recently been used as a recording device for recording dynamic images or a set-top box.
Generally, the magnetic disk device houses a single or plurality of magnetic disks and a recording/reproducing head. As the recording/reproducing head, a flying head is generally used wherein it moves above the magnetic disk at a constant flying height. Other than the flying head, the use of a contact head is proposed in order to make the distance to the medium closer.
The magnetic recording medium such as the magnetic disk is generally prepared by forming a NiP layer, a metal underlayer, a magnetic layer (a recording layer), a protective layer, a lubricant layer and so on on the surface of a substrate of aluminum alloy successively. Or, it is prepared by forming a metal underlayer, a magnetic layer (a recording layer), a protective layer, a lubricant and so on on the surface of a substrate such as glass successively. The magnetic recording medium includes a longitudinal magnetic recording medium and a perpendicular magnetic recording medium. In the longitudinal magnetic recording medium, longitudinal recording is generally carried out.
The tendency of making the magnetic disk more dense has been accelerated year by year, and various techniques for increasing the density have been proposed. For example, there is an attempt to increase the number of tracks by narrowing the space of information recording tracks. For example, a density of track of 100 ktpi or more is needed in order to realize 100 Gbit/inch2.
In each track, a controlling pattern for controlling synchronization and the position of the recording/reproducing magnetic head is formed. When the space of tracks is narrowed, it is necessary to make the magnetic pattern (hereinbelow, referred to as xe2x80x9cthe servo patternxe2x80x9d) used for controlling the position of the magnetic head dense in a radial direction of the disk, i.e., to form the magnetic pattern in more number, so as to conduct precise controlling.
Further, there is an increased demand to widen a data recording area to increase the data recording capacity by reducing the area other than that for recording data, namely, an area used for the servo pattern and gap portions between the servo areas and the data recording areas. For this purpose, it is necessary to increase an output of the servo pattern or to increase the accuracy of synchronization.
In the conventional technique, there has been widely used a method wherein an opening is formed in the vicinity of the head actuator of the drive (the magnetic recording device), a pin with an encoder is inserted into the opening to engage the actuator with the pin whereby the head is driven to a correct position to record the servo pattern. However, such method encountered difficulty in recording correctly the servo pattern because the position of the gravity center of the actuator was different from the position of the gravity center of a positioning mechanism, so that highly accurate track position control could not be obtained.
On the other hand, there is a proposed technique that laser beams are irradiated to a magnetic disk to deform locally the surface of the disk whereby projections and recesses are physically formed so that servo patterns of projections and recesses are produced. However, this technique had problems that the formed projections and recesses made the flying head unstable to affect adversely recording or reproducing information; laser beams having a large power was necessary in order to form the projections and recesses, thus being costly, and it took much time to form the projections and recesses one by one.
In view of the above, some servo pattern forming methods are proposed.
As an example, there is a method that a servo pattern is formed in a master disk having a magnetic layer of high coercive force, the master disk is brought to close contact with the magnetic disk, and an auxiliary magnetic field is applied from the outside whereby a magnetic pattern is printed (U.S. Pat. No. 5,991,104).
As another example, there is a method that a magnetic disk is previously magnetized along a certain direction; a soft magnetic layer having a high permeability and low coercive force is formed by patterning on a master disk, and the master disk is brought to close contact with the magnetic disk, and then, an external magnetic field is applied. The soft magnetic layer serves as a shield, and a magnetic pattern is printed to an unshielded area (see U.S. Pat. No. 3,869,711, EP915456 and xe2x80x9cReadback Properties of Novel Magnetic Contact Duplication Signals with High Recording Density FDxe2x80x9d (Sugita, R et. al, Digest of InterMag 2000, GP-06, IEEE).
In this technique, a master disk is used, and a magnetic pattern is formed in the magnetic disk by applying a strong magnetic field.
The intensity of a magnetic field generally depends on distances. When a magnetic pattern is recorded by applying a magnetic field, the boundary of the pattern is apt to be unclear due to a stray magnetic field. Accordingly, it is essential to bring the master disk to contact with the magnetic disk in order to minimize the stray magnetic field. As the magnetic pattern is finer, it is necessary to bring them close contact without any gap. Usually, the both members are press-contacted by using vacuum suction.
Further, the higher the coercive force of the magnetic disk is, the larger the magnetic field used for printing is, and accordingly, the stray magnetic field becomes large. Therefore, the perfect close contact is required.
Accordingly, the above-mentioned technique is easily applicable to a magnetic disk of low coercive force or a flexible floppy (trademark) disk which is easy to contact. However, it is very difficult to apply this technique to a magnetic disk for high density recording which has a coercive force of 3,000 Oe or more. Namely, in the magnetic disk comprising a hard substrate, there was a possibility that fine dust deposited thereon at the time of contacting closely whereby a defect was resulted in the magnetic disk, or an expensive master disk was damaged. In a case of a glass substrate, in particular, there was a problem that the deposition of dust might cause insufficient close contact, so that it might be impossible to conduct magnetic printing, or a crack was resulted in the magnetic disk.
On the other hand, in the technique described in Japanese Patent Application Nos. 2000-134608 and 2000-134611, a magnetic pattern is formed in a magnetic recording medium in the combination of heating a local portion and applying an external magnetic field. For example, a medium is magnetized previously in a certain direction; pulse-like energy beams or the like are irradiated to the medium through a patterned mask to heat the medium locally; an external magnetic field is applied while the coercive force of the heated area is reduced so that recording is conducted to the heated area by using the external magnetic field. Thus, a magnetic pattern is formed.
According to this technique, the recording is performed with a relatively weak external magnetic field because the coercive force is reduced by heating. Further, since the area for recording is limited to the heated area, and there is no possibility of recording to the area other than the heated area even when the magnetic field is applied thereto, a clear magnetic pattern can be recorded without bringing the mask to close contact with the medium. Accordingly, there is little possibility that the medium or the mask is damaged by the close contact and causing an increased defect in the medium. Further, this technique makes it possible to form a good oblique magnetic pattern which was difficult to form.
According to this technique, various kinds of fine magnetic pattern can be formed efficiently with high accuracy without damaging the medium or the mask, or increasing a defect in the medium.
In the magnetic recording media used recently have such problems that individual magnetic domains are small because the medium is recorded with high density and unwilled reversal is apt to occur due to a change of a heat or magnetic field whereby stability in recording may decrease.
In order to prevent this, it is necessary to make the grain size of crystal in the recording layer small so that the medium has a high coercive force and a high S/N. However, when the grain size of crystal is made small, the dynamic coercive force is apt to be large.
Further, in order to increase the thermal stability of magnetic domains, there has recently been proposed an AFC (anti-ferromagnetic coupled) medium which is formed by laminating two or more magnetic layers (a principal magnetic layer and a magnetic underlayer) by interposing a Ru layer or the like having a thickness of several Angstroms wherein the thermal stability of the principal magnetic layer is increased by coupling magnetically between these magnetic layers between which the Ru layer is interposed. Since this medium has a large apparent coercive force, a large magnetic field is required in order to reverse magnetization.
A so-called coercive force means generally a coercive force measured based on a B-H loop (a hysteresis loop) obtained when a magnetic field is sweeped over a long time of several 10 sec, i.e., a static coercive force. On the other hand, a dynamic coercive force is a coercive force obtainable from sweeping a magnetic field for a very short time such as scanning by a magnetic head. Usually, it means a coercive force obtained when a magnetic field intensity is changed for a short time of 1 sec. or less.
Generally, the dynamic coercive force is larger than the static coercive force. The dynamic coercive force varies largely depending on a time width in which the magnetic field is changed, and it indicates a large value as the time width is shorter. For example, the dynamic coercive force obtained by applying a magnetic field for 100 msec. is generally larger than the dynamic coercive force obtained by applying the magnetic field for 1 sec.
As described above, since a high S/N is essential in order to provide high density, it is inavoidable that the dynamic coercive force of such magnetic recording medium for high density recording is large. For example, in a magnetic recording medium having a recording density of 30 to 40 Gbit/inch2 level, the dynamic coercive force reaches twice or more as much as the static coercive force. For such medium, a magnetic head having a high writing ability is required even in a case of writing with the head.
When the magnetic layer of such medium is heated and magnetized with pulse-like energy beams for a very short time of several 10 nano sec in the above-mentioned magnetic pattern forming technique, the dynamic coercive force is involved dominantly. Namely, even when a magnetic field exceeding the static coercive force at the heating time is applied, the magnetic field intensity is insufficient, and a magnetic pattern having a sufficient output of signal may not be formed. Accordingly, it is necessary to apply an external magnetic field which can overcome the dynamic coercive force at the heating time to magnetize the magnetic layer.
In particular, in a case that an interference fringe of non-concentric circle generates in the medium by the reason that the distance between the mask and the medium is not constant or the like, there results a concentric circle area of strong light and a concentric circle area of weak light. In a dark area, the coercive force of the magnetic layer does not sufficiently decrease, and a large external magnetic field is needed for magnetization.
However, there is a way of applying a large external magnetic field with use of a permanent magnetic which generates usually magnetism. However, the permanent magnet may magnetize an area other than the heated area whereby a predetermined magnetic pattern is not obtainable; noises are generated in an area magnetized previously, or a magnetic pattern which has once been formed may be erased.
Therefore, the present invention is to provide a method for applying an effective external magnetic field to a magnetic recording medium having a high dynamic coercive force in the technique to form a magnetic pattern in the magnetic recording medium by combining the application of heat to a local portion and an external magnetic field.
It is another object of the present invention to provide a method for forming effectively a fine magnetic pattern in a magnetic recording medium for high density recording, an AFC medium or the like, a device for forming the magnetic pattern, a magnetic recording medium and a magnetic recording device capable of high density recording in a short time in an economical manner.
In accordance with a first aspect of the present invention, there is provided a method for forming a magnetic pattern by applying a first external magnetic field to a magnetic recording medium having a magnetic layer to magnetize uniformly the magnetic layer in a predetermined direction and heating locally the magnetic layer while a second external magnetic field is applied thereto, whereby the heated portion is magnetized in the direction opposite to the predetermined direction, wherein the second external magnetic field has a pulse-like magnetic field component.
According to a second aspect of the present invention, there is provided a magnetic recording medium in which a magnetic pattern is formed by the above-mentioned magnetic pattern forming method.
According to a third aspect of the present invention, there is provided a magnetic recording device characterized by comprising a magnetic recording medium in which a magnetic pattern is formed by the method for forming a magnetic pattern as described in the first aspect, a driving portion for driving the magnetic recording medium in a recording direction, a magnetic head having a recording portion and a reproducing portion, means for moving relatively the magnetic head with respect to the magnetic recording medium, and a recording/reproducing signal processing means which supplies a recording signal to the magnetic head and receives a reproducing signal from the magnetic head.
According to a fourth aspect of the present invention, there is provided a method for producing a magnetic recording medium having a magnetic layer, a protective layer and a lubricant layer formed on a substrate in this order wherein a magnetic pattern is formed in the magnetic layer, the method being characterized by comprising a step of forming the magnetic layer and the protective layer on the substrate, a step of forming the lubricant layer on the protective layer, and a step of applying a first external magnetic field to magnetize uniformly the magnetic layer in a predetermined direction, and heating locally the magnetic layer while applying a second external magnetic field having a pulse-like magnetic field component to magnetize the heated portion in the direction opposite to the predetermined direction whereby a magnetic pattern is formed.
According to a fifth aspect of the present invention, there is provided a magnetic recording medium prepared by the above-mentioned method.
According to a sixth aspect of the present invention, there is provided a magnetic recording device characterized by comprising a magnetic recording medium produced by the above-mentioned method, a driving portion for driving the magnetic recording medium in a recording direction, a magnetic head having a recording portion and a reproducing portion, means for moving relatively the magnetic head with respect to the magnetic recording medium, and a recording/reproducing signal processing means which supplies a recording signal to the magnetic head and receives a reproducing signal from the magnetic head.
According to a seventh aspect of the present invention, there is provided a magnetic pattern forming device for forming a magnetic pattern in a magnetic recording medium, the magnetic pattern forming device being characterized by comprising a medium holding means for holding the magnetic recording medium, an energy beam source for emitting energy beams, a projection means for projecting and irradiating the energy beams from the energy beam source to the magnetic recording medium, and a magnetic field generating means for applying a magnetic field to the magnetic recording medium, wherein the magnetic field generating means generates a magnetic field having a pulse-like magnetic field component.